Geoecological alteration of surface soils by the Hawaiian silversword (Argyroxiphium sandwicense DC.) in Haleakala's crater, Maui

Perez, F

The geoecological effects of organic matter addition to soil by Hawaiian silverswords (Argyroxiphium sandwicense DC., Asteraceae) in Haleakala's crater (Maui) are examined. Sets of 12 surface soil samples were gathered in three positions: bare soils; under the crown canopy of adjacent live plants; and below dead, withered rosettes. Silversword standing litter and live foliage were also collected. Several physical and chemical properties were investigated. Bare soils were coarse and exceptionally low in chemical nutrients and water-holding capacity. Litter addition below silverswords altered all properties except particle-size distribution; soils under dead rosettes became modified more strongly than those below live plants. While rosettes grow, standing litter is tightly preserved, thus only small amounts of organic matter are added to soil. When plants die, a substantial influx of litter is released in a short period (7 to 9 years average); this raises the C and N content, C/N ratios, exchangeable nutrient levels (Ca, Mg, K, P) and cation exchange capacity of soil. Physical properties, such as color, bulk density and porosity are also greatly affected; as a result, soil water-retention at field capacity nearly doubles under withering plants. Soil temperatures (5 cm depth) at noon were approximately 18.1 degrees C lower beneath silversword canopies (18.5 degrees C mean) than in contiguous bare soils (36.6 degrees C). This cooling may result in lower evaporation and further conservation of soil moisture, thus increased seed germination. Nutrient-retention strategies may have evolved in the semelparous Argyroxiphium to help it cope with the harsh, drought-prone soils and unpredictable climate of Haleakala. Silversword flowering results in a tremendous seed output, but the plant needs to ensure that some seeds--which have limited dispersal capability--will germinate, and thus provides a nutrient-rich, water-retaining substrate more suitable for seedling establishment than bare soils. Through its self-regulating influence on underlying soils, Argyroxiphium affects both soil formation and its own regeneration in Haleakala.

Geoecological alteration of surface soils by the Hawaiian silversword (Argyroxiphium sandwicense DC.) in Haleakala's crater, Maui

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